mtd: sm_ftl: Staticize local symbols

These local symbols are used only in this file.
Fix the following sparse warnings:

drivers/mtd/sm_ftl.c:25:25: warning: symbol 'cache_flush_workqueue' was not declared. Should it be static?
drivers/mtd/sm_ftl.c:44:9: warning: symbol 'sm_attr_show' was not declared. Should it be static?
drivers/mtd/sm_ftl.c:57:24: warning: symbol 'sm_create_sysfs_attributes' was not declared. Should it be static?
drivers/mtd/sm_ftl.c:110:6: warning: symbol 'sm_delete_sysfs_attributes' was not declared. Should it be static?
drivers/mtd/sm_ftl.c:574:5: warning: symbol 'sm_get_media_info' was not declared. Should it be static?
drivers/mtd/sm_ftl.c:881:17: warning: symbol 'sm_get_zone' was not declared. Should it be static?
drivers/mtd/sm_ftl.c:902:6: warning: symbol 'sm_cache_init' was not declared. Should it be static?
drivers/mtd/sm_ftl.c:912:6: warning: symbol 'sm_cache_put' was not declared. Should it be static?
drivers/mtd/sm_ftl.c:920:5: warning: symbol 'sm_cache_get' was not declared. Should it be static?
drivers/mtd/sm_ftl.c:931:5: warning: symbol 'sm_cache_flush' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>